In 1977, a robotic submersible called Alvin first located a black smoker on the ocean floor near the Galapagos Islands. Scientists knew that geysers existed on land and suspected that there must be some on the ocean floor as well.
Black smokers form when molten rock from the Earth's mantle pushes through the Earth's crust and out onto the seafloor; it then cools and creates more oceanic crust. The cooling process leaves cracks in the new crust through which cool water descends, collecting mineral traces from the rocks. The now mineral-rich water becomes superheated to around 700 degrees Fahrenheit and is forced back through the cracks in the crust. As this superheated, mineral-rich water comes in contact with the cold ocean water, the minerals separate from the water and become solid. This makes the water appear smoke-like.
Black smokers are most often found at points where the continental plates of the Earth's crust are moving apart. The action of the plates separating creates gaps through which sea water can seep and come into contact with magma.
